In a particular type of high-density optical scanning device, a solid immersion lens (SIL) is used to focus a radiation beam to a scanning spot onto an information layer of a record carrier. A certain distance between the exit face of the SIL and the outer face of the record carrier, for example 25 nm, is desirable to allow evanescent coupling of the radiation beam from the SIL to the record carrier. Evanescent coupling may otherwise be referred to as frustrated total internal reflection (FTIR). Such systems are known as near-field systems, deriving their name from the near field formed by the evanescent wave at an exit face of the SIL. An exemplary optical scanning device may use a radiation source which is a blue laser and emits a radiation beam having a wavelength of approximately 405 nm.
During scanning of the record carrier the evanescent coupling between the exit face of the SIL and the outer face of the record carrier should be maintained. An efficiency of this evanescent coupling may vary with a change in the distance of the gap between the exit face and the outer face. With an increase away from a desired gap distance the coupling efficiency will tend to decrease and consequently a quality of the scanning spot will also decrease. If the scanning function involves reading data from the record carrier, for example, this decrease in efficiency will result in a decrease in the quality of the data being read, possibly with the introduction of errors into the data signal.
In non near-field systems, such as compact disc (CD), digital versatile disc (DVD) or Blu-ray, it has been known that tilt misalignment of the record carrier with respect to an optical axis of an objective lens system can adversely affect quality of the scanning spot during writing to and reading from the information layer.
Changes in tilt misalignment may be attributed to an unevenness of a planarity of the record carrier. This may be due to warping of the disc, possibly due to environmental factors such as high temperatures over time or to a low quality manufacturing process of the record carrier. Alternatively, or in addition, tilt misalignment may be caused by poor clamping of the record carrier within the scanning device.
For optical scanning devices which are not of a near-field type, systems are known which allow a tilt misalignment of a record carrier to be measured and corrected for. One conventional system involves using a tilt detector to detect a tilt misalignment of the record carrier and to correct the tilt misalignment based on the extent of the detected tilt misalignment. A different conventional system involves performing an optimisation routine during which data is first written to the record carrier and then read. The quality of the read data is then determined as a function of the tilt misalignment. This allows the tilt misalignment to be corrected for, if necessary.
Use of a conventional tilt detector for detecting tilt misalignment of the record carrier would not provide a sufficient level of accuracy in a near-field system due to the very small tolerances involved, and further would require a high degree of alignment of the objective system with respect to the tilt detector. Exceeding such small tolerances may lead to contact between the SIL and the record carrier, possibly damaging the SIL and/or the record carrier.
Use of an optimisation routine, similar to the known systems, for measuring and estimating tilt misalignment in a near-field system would not be practical.